This invention relates to an apparatus and method for forming fine liquid metal droplets. More particularly, it relates to an apparatus and method for forming fine liquid metal droplets wherein the desired average size and dimensional spread about the average size of the droplets can be controlled.
Ideally, an apparatus and method for forming droplets from liquid metal would be capable of producing fine or small metal droplets, e.g. 40 microns in diameter, providing continuous control over droplet size, and generating uniformly-sized droplets. It is also desirable to produce a quantity of droplets to support a commercial or industrial application.
The properties of many advanced alloys depend on the rate at which the metal is cooled. A known method for obtaining high cooling rates comprises generating liquid metal droplets and then exposing them to a cool gas stream. Another known method for obtaining high cooling rates, known as splat-cooling, comprises generating liquid metal droplets and directing them to impact on a cooled surface. The resulting cooled droplets from either method form a powder which may be sintered or hot pressed to form articles.
It has been found that smaller, fine droplets can have superior properties if they can be cooled fast enough to prevent component segregation within the droplet, since a substantially homogeneous structure will result. Such homogeneity will provide intrinsic strength to the finished article. However, the droplets must not be so small as to have too high a surface area per unit volume because a large amount of oxidation may occur, resulting in excessive formation of surface impurities, such as oxides, which become sites for metal fatigue. Also the droplets must not be too large as they become too hard to cool or quench rapidly and also act as fatigue sites in an otherwise relatively homogeneous structure.
The production of small droplets has been addressed in a number of areas. A. D. Moore in "Electrostatics", Scientific American, Vol. 226, March 1972, discloses an apparatus for producing paint droplets which uses a spinning bell-shaped member having an electrical charge. Paint at room temperature and pressure is fed to the center of the bell and flows as a film toward the edge where it is subjected to an electric field and forms uniformly spaced streamers. Paint typically has a water or oil base. Thus, the surface tension, i.e. the force which must be overcome to form fine metal droplets, of a liquid or molten metal is significantly greater than that of paint and therefore the strength of the electric field required to overcome this force to form fine metal droplets would cause electrical arcing or dielectric breakdown of the gas at room temperature and pressure. Further, because paint is able to be handled at room temperature, the stresses experienced by the spinning member are unlikely to substantially damage or deform the member.
In "Rapid Solidification Effects of Micron-Size Droplets" by M. R. Glickstein et al., published in Rapid Solidification Processing, R. Mehrabian et al. eds., Claitor's Publishing Division, Baton Rouge, 1978, a disk spinning at the rate of 24,000 RPM and disposed in helium at atmospheric pressure is described as being used to form droplets from liquid metal. The droplets display a relatively large size distribution as shown in FIG. 3 therein, where less than 20% of the droplets produced are less than 50 microns in diameter.
In "Electrohydrodynamic Generation of Submicron Particles for Rapid Solidification", by J. Perel et al., published in Rapid Solidification Processing, 1978, a capillary tube disposed in a vacuum chamber and having an electric field at one end is described for use in producing liquid metal droplets. The apparatus is intended to be a laboratory set up for generating metal droplets which will be subjected to analytical testing. The relatively small (75 micron) diameter capillary output orifice could be impractical from a commercial standpoint as the molten metal source would have to be substantially free of impurities to prevent fouling of the orifice. Further, the output quantity of 20 grams per day desired by Perel et al. would generally be inadequate to support a commercial operation.
Another method for forming droplets from liquid metal uses high velocity gas jets to atomize a stream of the liquid metal. However, the droplets so formed are generally larger and have a greater size spread than those formed by the other methods previously discussed.
Because the methods as hereinbefore mentioned produce a relatively large spread of droplet sizes, a screening step is typically employed to sort out the undesired sizes. Such handling can lead to impurities being added to the droplets. If smaller, uniformly sized droplets could be produced, the screening step could be eliminated.